Immune Reconstitution after Autologous Stem Cell Transplantation for Multiple Myeloma

Background: High-dose chemotherapy followed by autologous stem cell transplantation (ASCT) for multiple myeloma (MM) offers a unique opportunity for the early introduction of consolidative immunotherapy to improve patient outcomes.  Post-transplant reconstitution of immune cell subsets, however, occurs with disparate kinetics that can affect efficacy.  A comprehensive understanding of the immunologic milieu is therefore essential to the rational development of immunotherapeutic interventions after ASCT, where relapse remains the primary cause of treatment failure.

Methods: Immune reconstitution in 40 MM patients undergoing ASCT was evaluated for one year.  Peripheral blood from patients was obtained before ASCT and on d +11, +30, +90, +180, and +365 after ASCT.  Mononuclear cells were analyzed by flow cytometry for phenotypic assessment of lymphocyte subset composition. Functional assessment of T cell responsiveness was assayed as follows: Monocyte-derived dendritic cells (moDCs) generated from peripheral blood were electroporated with WT1 mRNA and then added in serial doses to triplicate microwells containing autologous T cells obtained pre- and post-ASCT (d +11, 30, 90), supplemented with IL15 x 7d.  Antigen-specific target cell lysis by cytotoxic T lymphocytes (CTLs) induced by moDCs was evaluated using a flow cytometry-based assay.

Results: Total CD3⁺ T cell levels return to normal by d +30 (Fig 1A).  The recovery of CD8⁺ T cells, however, precedes the recovery CD4⁺ T cells, resulting in an inverted CD4/CD8 ratio (Fig 1A).  CD4⁺CD45RO⁺ memory T cell recovery occurs by d +11, whereas CD4⁺CD45RA⁺ naļve T cells remain low at one year (Fig 1B).  Immune-suppressive CD3⁺CD4⁺CD25^bright CD127^neg regulatory T cells (Tregs) are reduced early post-nadir as CD8⁺ T cell recovery occurs, resulting in a markedly reduced Treg:CD8⁺ effector T cell ratio (Fig 1C).  T cells uniformly express the negative regulatory molecules, CTLA4 and PD1, after ASCT (data not shown).  CD3^negCD56⁺ natural killer cells exhibit rapid and sustained recovery after ASCT (Fig 2).  Robust antigen-specific CTLs are induced by autologous mRNA-electroporated moDCs after only 7 days' stimulation of T cells in vitro, demonstrating that active cellular immune responses can be elicited as early as d +11 (Fig 3).  Subgroup analysis of patients based on pre- and post-ASCT disease status (i.e., PR vs VGPR vs CR) revealed no significant differences in the pattern of immune reconstitution between groups.

Conclusion: In addition to decreased disease burden, a favorable shift in Treg to effector T cell ratios characterizes the early post-transplant period.  This provides a critical window for immunotherapeutic modalities like vaccines and checkpoint blockade agents to induce antitumor immunity.